Surgical and other medical procedures are often undertaken that involve delivery and implantation of tubular stents or grafts within organs, cavities or body lumens including blood vessels, urinary tracts, esophageal tracts, respiratory tracts, bile ducts, and colo-rectal tracts, that function to maintain a lumen for passage of body fluids or blood therethrough. Such stents are employed typically to widen or maintain the width of such a body lumen where an obstruction, injury or disease process threatens to close it.
For example, percutaneous transluminal coronary angioplasty (PTCA) is used to increase the lumen diameter of a coronary artery partially or totally obstructed by a build-up of cholesterol fats or atherosclerotic plaque. Typically a first guidewire of about 0.035 inches in diameter is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the first guidewire to a point just proximal of the stenosis. The first guidewire is then removed. A balloon catheter on a smaller 0.014 inch diameter second guidewire is advanced within the guiding catheter to a point just proximal of the stenosis. The second guidewire is advanced into the stenosis, followed by the balloon on the distal end of the catheter. The balloon is inflated causing the site of the stenosis to dilate. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten re-closure of the dilated vessel or even perforations in the vessel wall after the balloon catheter is deflated and removed. Implantation of a stent can provide support or "scaffolding" for such flaps and dissections and thereby prevent re-closure of the vessel or provide a patch repair for a perforated vessel wall until corrective surgery can be performed. It has also been shown that the use of intravascular stents can measurably decrease the incidence of a later restenosis after angioplasty thereby reducing the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
Intravascular stents and grafts can also be used to repair aneurysms, to support artificial vessels as liners of blood vessels or to repair dissections of blood vessels. The vascular passageways maintained open by stents range in diameter from small coronary vessels of 3 mm or less to 28 mm in the aortic vessel. Moreover, as noted at the outset, suitably sized stents have been proposed or clinically employed to maintain the patency of other body lumens, including the vas deferens, ducts of the gallbladder, prostate gland, trachea bronchus and liver.
A wide variety of stent designs have been proposed and/or clinically employed in such body lumens including solid tubular metal or plastic tubes, coiled wire stents, mesh stents, fabric grafts, and the like, that may be either permanently or temporarily implanted. The stent is typically delivered to the site of implantation in a small diameter, compressed state and then radially expanded to an expanded state against the body vessel wall. Depending upon the design, the stent can be made to be radially self-expanding in situ when released from a delivery catheter or the like or expandable by the use of an expansion device, e.g., a balloon catheter, operated from outside the patient's body. A variety of expandable stents that employ an expanding balloon, including typical single coil wire expandable from a delivered diameter to an expanded diameter are disclosed in U.S. Pat. No. 4,886,062 to Wiktor and in U.S. Pat. No. 5,201,901 to Harada et al.
Self-expanding, single coil, wire stents are disclosed in commonly assigned U.S. Pat. Nos. 5,372,600 to Beyar et al. and 5,246,445 to Yachia et al and in U.S. Pat. No. 5,797,952 to Klein. Such single coil wire stents are characterized as "open coils" having substantial apparent spacing between adjacent coil turns when in the expanded state or as "closed coils" having insubstantial spacing between or actual contact of adjacent coil turns. Single filar, open coil, wire stents that range from 15 mm to 150 mm in length and 2 mm to 12 mm in diameter when released and self-expanded in situ have been sold or proposed by the assignee of the present invention in various CardioCoil.TM. and VascuCoil.TM. models for coronary and peripheral vascular implantation. Further single, closed coil, wire stents that range from 40 mm to 150 mm in length and 6 mm to 30 mm in diameter when released and self-expanded in situ have been sold or proposed by the assignee of the present invention in various EndoCoil.TM., EsophoCoil.TM., CoRectCoil.TM., UroCoil.TM., and ProstaCoil.TM. models for implantation in the bile duct, esophagus, colo-rectal tract and urinary tract.
Use of certain of these stents is described by Beyar et al. in Cath. Cardiovasc Diag., 32:162-170 (1994). In each case, the coil stent is delivered by winding it down into a smaller diameter and fixing it onto a stent delivery catheter as also explained in the above-referenced '445 and '600 patents. When the stent is positioned at the desired site, the ends of the stent are released from the catheter, and the single filar coiled wire stent self-expands by its spring force to the specified released stent diameter and length dimensions.
These single filar stents are formed of a single filar wire having a circular cross-section or a rectangular or "ribbon shaped" cross-section (although other cross-section shapes are suggested in the above-referenced '600 patent) terminating with enlarged "ball tips" at the wire ends. The ribbon shaped wire is preferred because it can be formed of many desirable materials, e.g., superelastic or pseudoelastic alloys such as disclosed in U.S. Pat. No. 5,597,378 to Jervis, and results in a "low profile" or thin stent scaffolding that depends on the ribbon shaped thickness. In addition, ribbon shaped wire can achieve stent strength equivalent to stents of the same stent diameter and coil pitch made of round wire but employing a thickness that is thinner than the round wire diameter, thus yielding smaller strains when wound down. The ribbon shaped wire can be therefore wound down to a smaller diameter about a stent delivery catheter without exceeding its strain limit and suffering plastic deformation than the comparable circular cross section wire. If the strain is too large, the material will experience plastic deformation to such an extent that the stent will not recover to the intended length and diameter dimensions following release and deployment.
A self-expanding, double spiral coil, wire stent and delivery catheter are disclosed in U.S. Pat. No. 5,772,668 to Summers et al. The double spiral coil is formed of a continuous loop having a pair of parallel legs that are joined together at each end of the legs by U-shaped, semicircular, end cusps. The legs formed into the continuous loop are wound into a coil of constant pitch and common diameter between the opposite end cusps. In this way, each leg remains in parallel to the other leg, and in some instances the end cusps project away from one another and in the axial direction of the coil. The inner diameters of the semicircular cusps define the spacing between the legs of the loop.
The double spiral coil stent is wound about the delivery catheter in the reduced diameter so that the adjacent legs do not overlap one another. In one embodiment, the distal ends of retention wires that extend through the hollow lumens of the delivery catheter are extended through the interior loops of the end cusps to retain the double spiral coil stent compressed about the catheter surface during introduction to the selected site. The retention wires are retracted from the interior loops to release the double spiral coil stent, allowing it to expand at the implantation site. In other embodiments, the end cusps are extended axially to hook the U-shaped cusp ends over radially projecting pins or cams, and the stent delivery catheter is manipulated to release the cusp ends. In this case, the stent ends are stressed axially so that the axial spring force applied through the cusp ends to cams overcomes the radial spring force that would cause the stent ends to release from the cams.
The use of the end cusps and legs constructed in this manner and these retention mechanisms require that the end cusps and short sections of the legs coupled thereto to be bent or twisted to fit the retention mechanisms of the stent delivery catheter through the cusp ends during implantation. The stents of the '668 patent are described as being formed in several ways, but the depicted stents appear to be formed of substantially circular cross-section wire that is first formed into a loop defining the two legs and end cusps described above and then wound into a stent coil. The deformations of the stent ends to be attached to the retention and release mechanisms of the stent delivery catheter is best accomplished when the stent cross-section is substantially circular because it is difficult to bend ribbon shaped wire against its width direction. Thus, the strain resistance and stent profile advantages of ribbon shaped wire cannot be realized using the loop defining the two legs and end cusps described above.
Moreover, use of the end cusps to couple the legs together and the constant spacing between the two parallel legs limits the distance that the legs can be spaced apart in the pre-formed loop that is wound to form the double coil stent of the '668 patent. If the cusp end diameter is increased to accommodate a greater spacing between the legs, then the projections of the end cusps in the direction of the stent coil axis is increased in severity. Therefore, the leg spacing defined by the end cusp diameters either limits the pitch of the spiral dual coil stent to maintain a constant distance or spacing between the wire coil turns or causes the resulting adjacent stent coil turns to have a spacing that is wider than the spacing between the parallel legs as shown in the figures of the '668 patent. As a result, the uneven, alternating wide and narrow spacing of adjacent coil turns causes the support or "scaffolding" of the body lumen to be uneven.
Finally, it is frequently desirable to be able to retrieve a deployed and expanded stent if it is not deployed optimally. The end cusps of the double spiral coil stent of the '668 patent would be difficult to snag to retrieve it and retract it into the lumen of a catheter.